The present invention relates to an edge-emitting semiconductor laser device, and particularly to a semiconductor laser device capable of realizing stable operation with a high output over a long period of time by suppressing degradation of an emitting edge face due to catastrophic optical damage.
In a GaAs based edge-emitting semiconductor laser device, there occurs a phenomenon that along with the increased injection current in order to enlarge an optical output, the optical output is rather abruptly decreased. This is due to catastrophic optical damage (COD) at an emitting edge face of the semiconductor laser device. It is known that such COD may occur on the basis of the following mechanism.
A high density surface level is present on an emitting edge face of a semiconductor laser device, and accordingly, when a current is injected in the semiconductor laser device, a non-radiative recombination current flows via the surface level. As a result, the carrier density near the emitting edge face becomes lower than that in the laser, to yield light absorption. The light absorption causes heat generation, to raise the temperature of a portion near the emitting edge face, so that a band gap energy near the emitting edge face is reduced, to further increase the optical absorption. The positive feedback loop associated with optical absorption and heat generation significantly raises the temperature of the emitting edge face, which finally leads to fusing of the emitting edge face, thereby stopping laser oscillation. It is also known that optical absorption may be increased by oxidation of the emitting edge face and occurrence of point-defects such as vacancies at the emitting edge face.
To prevent occurrence of such COD at an emitting edge face of a semiconductor laser device, a countermeasure has been made to output laser light from the emitting edge face as much as possible by forming a low reflection film on the emitting edge face.
Japanese Patent No. 2870486 (hereinafter, referred to as “first related art”) discloses a semiconductor laser device, wherein a low reflection two-layer film having an Al2O3 film and an Si3N4 film is provided on en emitting edge face. This document describes that since the direction of the stress due to strain caused in the Al2O3 film is reversed to the direction of the stress due to strain caused in the Si3N4 film, the stress due to strain caused in the Al2O3 film is canceled by the stress due to strain caused in the Si3N4 film, to thereby eliminate the stress due to strain in the stacked film having the Al2O3 film and the Si3N4 film.
The above document further describes that since the stress due to strain caused in the emitting edge face of the semiconductor laser device can be reduced by providing the two-layer film having the Al2O3 film and the Si3N4 film on the emitting edge face, it is possible to suppress occurrence of point-defects such as vacancies due to strain caused by the stress at the emitting edge face, and hence to form a stable dielectric-compound semiconductor interface, thereby suppressing degradation of the emitting edge face due to COD.
Japanese Patent Laid-open No. Hei 6-224514 (hereinafter, referred to as “second related art”) discloses a semiconductor laser device configured such that a two-layer film including an Si film having a relatively high thermal conductivity and an Al2O3 film having a relatively low thermal conductivity is provided as a high reflection coating film on a rear edge face of the semiconductor laser device, wherein the thermal conductivity of the entire high reflection coating film is increased by making the thickness of the Al2O3 film thinner than λ/n1 (λ: wavelength, n1: refractive index of Al2O3) and making the thickness of the Si film thicker than λ/n2 (λ: wavelength, n2: refractive index of Si), to improve the heat dissipation performance of the semiconductor laser device.
The above-described first related art, however, has the following problems.
The semiconductor laser device according to the first related art is intended to suppress degradation of the emitting edge face due to COD by canceling the stress due to strain caused in the emitting edge face by providing the stacked film having the Al2O3 film and the Si3N4 film thereon.
Accordingly, the thicknesses of the Al2O3 film and the Si3N4 film and the film formation conditions thereof are required to be set such that the edge reflectance against a specific emission wavelength becomes a specific value on the basis of a refractive index (n=1.6) of Al2O3 and a refractive index (n=2.1) of Si3N4, and that the stress due to strain caused in the emitting edge face is canceled.
In actual, however, it is very difficult to select the thicknesses of the Al2O3 layer and the Si3N4 layer and the film formation conditions thereof so as to satisfy both the edge reflectance and the canceling of the stress due to strain. An additional problem is that since the degree of freedom in film formation conditions of the Al2O3 layer and the Si3N4 layer is small, the film formation thereof is very difficult.
As a result, from the viewpoint of practical use, it is difficult to apply the first related art to a variety of semiconductor laser devices different from each other in terms of emission wavelength, edge reflectance, and application use.
The first related art has another problem that since Si3N4 poor in chemical and thermal stability is exposed, the resistance against chemicals at the subsequent processing steps, for example, a cleaning step becomes low, to degrade the fabrication yield, and the optical characteristic is varied depending on the progress of oxidation due to atmospheric air accompanied by drive of the semiconductor laser device.
A further important problem of the semiconductor laser device provided with the low reflection multi-layer film according to the first related art is that the laser characteristic is easy to be degraded, thereby making it difficult to realize stable operation over a long period of time.
The second related art, which is intended to provide the high reflection film on the rear edge face of the semiconductor laser device, is technically difficult to be applied to the emitting edge face of the semiconductor laser device.
If the second related art is applied to the emitting edge face, since the Si film causes optical absorption for light having a wavelength in a 0.8 μm band or less, the emission wavelength range of the semiconductor laser device to which the second related art is applied is limited.